This invention relates to an image processing apparatus having a functional feature of smoothing the profile of an image. It also relates to an image processing method and to an image-forming apparatus realized by using such a method.
An image processing apparatus, which may be a copying machine, optically reads the original image and forms an electrostatic latent copy image of the original on a photosensitive dram, which operates as image carrier. Then, the electrostatic latent image is visualized by means of a developer and transferred onto a sheet of paper.
A smoothing technique is typically used for processing images in a copying machine. With this technique, the image is divided into a predetermined number of pixel blocks, which are sequentially sampled to produce a pixel matrix. The produced pixel matrix then provides sampled image data. Each of the sampled image data is then compared with a set of reference image data, which are also referred to template image data and contained in a look-up table that is stored in advance in the copying machine. If the sampled image data finds a matching reference data, data for black or white pixels are appropriately added to the profile of the black image that is represented by the sampled image data and expressed by the corresponding coordinate values in order to produce a smoothed profile that is free from jags.
FIG. 21 of the accompanying drawing illustrates the effect of a smoothing technique. Known smoothing methods include the one disclosed in Japanese Patent Application TOKKAI No. 7-57102.
While a smoothing technique may suitably be used on characters, it is not adapted to photographic images showing delicate tonal gradations because intermediate gradations are adversely affected by the smoothing effect to damage the quality of the copied image.
In order to cope with this problem, copying machines are currently adapted to discriminate character regions and photograph regions of images so that the photograph regions may be exempted from smoothing.
When an image is binarized by means of a pseudo-gradation technique such as error propagation, a very small island-like region comprising a set of black pixels and contained in a photograph region can often be mistaken for that of a character region.
Then, the photograph region is automatically subjected to a smoothing process to consequently damage the quality of the photograph.
More specifically, with error propagation, an intermediary gradation is expressed by the average of the gradations of a plurality of pixels (referred to as areal gradation). When a photograph region that has been processed by error propagation is smoothed, the gradation expressed by the average of the gradations of a plurality of pixels can be differentiated, if slightly, from the real gradation of the region to consequently damage the quality of the photograph. Such differentiation can result in a noise pattern of black spots appearing in intermediately gradated areas of an image as schematically illustrated in FIG. 22 of the accompanying drawings. As schematically illustrated in FIG. 23, such a black spot of a noise pattern is given rise to when one or more than one new pixels are added to a (photograph) region that is to be expressed by a predetermined number (four in FIG. 23) of pixels.